Data on novel C fibers@MoSe2 nanoplates core–shell composite for highly efficient solar-driven photocatalytically degrading environmental pollutants

The data presented in this article are related to a research article entitled ‘Highly efficient solar-driven photocatalytic degradation on environmental pollutants over a novel C fibers@MoSe2 nanoplates core–shell composite’ (Wang et al., 2018) [1]. In this article, we report original data on the synthesis processes optimization of the proposed composite together with its formation mechanism. The report includes the composition, microstructure and morphology of the corresponding samples, and the photocatalytic activity and stability of the optimal composite. Compared with commercially available MoSe2 powder, the reaction rate constant of the optimal composite catalyst for the degradation of methylene blue (MB) and rhodamine B (RhB) under simulated sunlight irradiation (SSI) could be increased in a factor of about 14 and 8, respectively. The data are presented in this format to allow the comparison with those from other researchers in this field, and understanding the synthesis and photocatalysis mechanism of similar catalysts.

The designed experiments included the optimization of synthesis processes and comparison on the photocatalytic degradation of MB, RhB, p-chlorophenol (4-CP) and K 2 Cr 2 O 7 (Cr, VI) Data source location The composite was grown in Beijing, China

Data accessibility
The data are available with this article

Value of the data
The data on the synthesis processes optimization of the C fibers@MoSe 2 nanoplates core-shell composite (NPCSC) could give an insight into its formation and photocatalysis mechanisms to other researchers interested in the synthesis and application of photocatalysts.
The data can be used by researchers interested in developing other composite photocatalysts and understanding their photocatalytic mechanism.
The data can be used by researchers interested in developing new energy materials, and energy storage and conversion devices.

Data
The data presented in this paper are related to a research article entitled 'Highly efficient solardriven photocatalytic degradation on environmental pollutants over a novel C fibers@MoSe 2 nanoplates core-shell composite' [1].
It includes data on the synthesis processes optimization and formation mechanism of the present C fibers@MoSe 2 NPCSC (Figs. [1][2][3][4][5], which reveal that numerous MoSe 2 thin nanoplates are grown insitu, densely and even vertically on the surface of the C fibers, forming the optimal core-shell composite. Data on the photocatalytic performance and stability of the optimal composite catalyst are also presented (Figs. [6][7][8][9][10][11][12][13][14]. In addition, data on the activity for the photodegradation of 4-CP and Cr (VI) over the present C fibers@MoSe 2 NPCSC and other photocatalysts are compared in Tables 1 and 2.

Experimental design, materials and methods
Novel highly efficient C fibers@MoSe 2 NPCSC photocatalyst for environmental remediation was described by Wang et al. [1]. In order to improve the photocatalytic performance of the composite, the synthesis processes were optimized by changing the reaction temperature from 900 to 1100°C, and adjusting the applied amounts of MoO 3 powder from 1.0 to 1.6 g in 5 mL absolute ethanol and Se  powder from 0.5 to 3.0 g. All the prepared samples were characterized by XRD and SEM. And all the experiments were conducted in duplicates.
From Fig. 1, it can be seen that the sample prepared at 900°C almost consists of pure MoO 2 nanoparticles. As the temperature increased, MoO 2 nanoparticles were further, gradually reduced into     This figure reveals that when 1.0 g of MoO 3 powder was used, the desirable product had MoSe 2 nanoplates in a quite high density, and the by-products such as MoO 2 were in the least amount. Fig. 3 reveals that, under the present condition, when the applied amount of Se powder increased up to 3.0 g, the sample had been a pure C fibers@MoSe 2 NPCSC.
The recorded EDX spectrum on the outer shell indicates that it consists of mainly Mo and Se atoms with very little of C atoms (see Fig. 4a). In combination with its morphology, it can be seen that the outer shell is composed of molybdenum selenide nanoplates. In addition, a small amount of C atoms was also detected on the molybdenum selenide nanoplates. The EDX spectrum on the inner core reveals that it is composed of only C atoms (see Fig. 4b), implying that the inner core is of elemental carbon.
Based on the results presented in Figs. 1 and 2 in Ref. [1] and the corresponding discussion, in combination with the present Figs. 1-4, a possible formation mechanism called in-situ "symplastic growth" can be used to explain the growth of the present C fibers@MoSe 2 NPCSC. The whole process can be schematically shown in Fig. 5. In the first step, a composite of PAN fibers@MoO 3 particles was formed by soaking the PAN fibers in MoO 3 suspension, where the MoO 3 particles were uniformly coating on the surface of the PAN fibers. In the second step, at 400-600°C under the action of inert gas, the oxygen-containing functional groups of the pre-oxidized PAN fibers were dehydrated and cross-linked to form a more stable trapezoidal structure. The trapezoid molecules were connected into a graphene-like structure by the dehydrogenation reaction. When the temperature raised up to above 600°C, in the third step, denitrification reaction would occur, forming structured C fibers and releasing H 2 , NH 3 , HCN, H 2 O and so on [2]. Synchronously, the partially pyrolyzed C reacted with MoO 3 to produce MoO 2 and reducing gas CO. As the amounts of reducing H 2 and CO gases increased, and more Se vapor was fed from the upstream, MoO 2 nanocrystals were further selenized to form MoSe 2 nanoplates, finally producing the C fibers@MoSe 2 NPCSC. Fig. 6 reveals that without photocatalysts, MB is self-sensitized but RhB is stable under SSI. Under the present conditions, MB will be decolourized by SSI at about 10%, but without photocatalysts, no photodegradation under SSI could be observed on RhB. Fig. 8 shows the decolourization effects on MB under SSI over the as-prepared C fibers@MoSe 2 NPCSC and commercially available MoSe 2 powder, respectively. As is seen from Fig. 8a, a dark adsorption for 60 min was performed prior to light irradiation so as to reach the adsorption-desorption equilibrium. In this stage, the decolourized MB over the C fibers@MoSe 2 NPCSC was 25.9%, while that over the MoSe 2 powder was only 7.3%. During the photocatalytic degradation, the degraded MB over the C fibers@MoSe 2 NPCSC reached 19.2%, whereas that over the commercially bought MoSe 2 powder was only 1.9%. This result reveals that the commercially bought MoSe 2 powder has no usable photocatalytic activity on the degradation of MB; however, after compositing with C fiber, the photocatalytic performance of MoSe 2 nanoplates can be greatly enhanced. The photocatalytic degradation of MB follows the pseudo-first-order kinetics as described by the equation of −ln(C/C 0 ) ¼kt [3,4]. Through this equation, straight lines can be fitted into Fig. 8b, in which the slope of the straight lines can be explained as the photocatalytic reaction rate constant k. The rate constants of the photodegradation reactions on MB over the as-prepared C fibers@MoSe 2 NPCSC and commercially bought MoSe 2 powder are 0.0043 and 0.0003 min −1 , respectively. This result indicates that after compositing with C fibers in the form of the present C fibers@MoSe 2 NPCSC, the rate constant of MB photodegradation over MoSe 2 nanoplates under SSI was increased in a factor of about 14. Fig. 8c illustrates the photocatalytic activity of the sample stored for 4 months on degrading MB under SSI. It is seen that the totally decolourized MB by the catalyst stored for 4 months was 57.3%, almost equaling to that by the fresh one (54.9%). This result reveals an excellent structural stability of the C fibers@MoSe 2 NPCSC photocatalyst for a long period of storage.
Moreover, the sample was also repeatedly used for the photodegradation of MB to further evaluate its chemical stability. The result is displayed in Fig. 8d. It was revealed that during the repeated use, the photocatalytic activity of the catalyst for the degradation of MB under SSI decreased very little after 3 times of experiments were carried out. This result in combination of their well-kept morphology and composition as shown in the SEM images in Fig. 9 after being used indicates that such catalyst has a relatively high stability during photocatalytic application. As for the very little reduction in photocatalytic activity during recycling use, it might be resulted from the exfoliation and loss of a   few MoSe 2 nanoplates from the sample during the repeated washing and drying after each cycle of photocatalytic test.
In combination with its original morphology and composition, Fig. 9 reveals that the morphology and composition of the catalyst can be well maintained during the photocatalytic degradation of MB, indicating that the catalyst has good stability during such reactions.
It can be seen from Fig. 10a that in the dark adsorption stage, the decolourized RhB by the C fibers@MoSe 2 NPCSC was 11.2%, while that by the commercially available MoSe 2 powder was only 1.4%. In the photocatalytic stage, the degraded RhB over the C fibers@MoSe 2 NPCSC reached 18.9%, but that over the commercially bought MoSe 2 powder was only 2.8%. This result reveals that the commercially available MoSe 2 powder has no photocatalytic activity for the degradation of RhB. However, the photocatalytic activity of MoSe 2 nanoplates can be greatly enhanced after compositing with C fiber in the form of the reported C fibers@MoSe 2 NPCSC. On the basis of the recorded data on the photocatalytic degradation reactions, straight lines can be fitted for the plots of -ln(C/C 0 ) versus irradiation time, and the results are shown in Fig. 10b. From the fitted graph, the rate constants of the photodegradation reaction on Rhb over the as-prepared C fibers@MoSe 2 NPCSC and commercially available MoSe 2 powder were calculated as 0.00347 and 0.00043 min −1 , respectively. It is seen that after composting with C fibers, the photodegradation rate of RhB over the present C fibers@MoSe 2 NPCSC was 8 times higher than that over the commercially available MoSe 2 powder. Fig. 10c reveals the stability of the C fibers@MoSe 2 NPCSC on degrading RhB under SSI. While the other conditions were fixed, the decolourized RhB by the C fibers@MoSe 2 NPCSC catalyst stored for 4 months reached 72.6%, which is very close to that over the fresh C fibers@MoSe 2 NPCSC (69.9%). This result indicates that after being stored for a long time, the fibers@MoSe 2 NPCSC still had good photocatalytic performance on the photodegradation of RhB. Fig. 10d displays the photocatalytic repeatability of the C fibers@MoSe 2 NPCSC on degrading MB under SSI. Three repeated tests were performed on the photodegradation of RhB over the same catalyst sample. It is seen from this graph that, the photocatalytic activity of the C fibers@MoSe 2 NPCSC on the degradation of RhB decreased very little after each test. In combination with their good morphology and well-kept composition after photodegradation test as shown in Fig. 11, it was revealed that the present C fibers@MoSe 2 NPCSC had excellent photocatalytic stability.
In combination with its original morphology and composition, Fig. 11 reveals that the catalyst could maintain its morphology and composition during the photocatalytic degradation of RhB, indicating that the C fibers@MoSe 2 NPCSC catalyst has good stability during such reactions.
In combination with its original morphology and composition, Fig. 13 reveals that the catalyst could maintain its morphology and composition during the photocatalytic degradation of 4-CP, indicating that the C fibers@MoSe 2 NPCSC catalyst has good stability during such photocatalytic reactions.
In combination with its original morphology and composition, Fig. 14 indicates that the morphology and composition of the catalyst can be well maintained during the photocatalytic degradation of Cr(VI), indicating that the catalyst has good stability during such photocatalytic reactions.